Amplifier circuit for AM broadcasting

ABSTRACT

An amplifier circuit for AM broadcasting for amplifying an inputted AM broadcast signal by an FET and outputting it. The amplifier circuit comprises FETs for signal amplification which are P-channel MOSFETs ( 4, 5 ) of relatively small flicker noise. While suppressing the flicker noise to a lowest possible level, more circuits including the RF amplifier for AM broadcasting can be integrated on one chip, thereby realizing small size and low noise of the circuits.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an amplifier circuit for AMbroadcasting, and more particularly, to one preferably applicable to anRF (Radio Frequency) amplifier used for a broadcast signal input stageof a radio receiver that receives AM broadcast signals, etc.

[0003] 2. Description of the Related Art

[0004]FIG. 1 shows a configuration of a conventional AM broadcastingreception circuit. FIG. 1A shows a configuration of a tuning circuitformat and FIG. 1B shows a configuration of a non-tuning circuit format.As shown in FIG. 1A, the conventional AM broadcasting reception circuitin the tuning circuit format is constructed of a capacitor 101, aresistor 102, an FET (Field Effect Transistor) for signal amplification103, a tuning circuit 104 and an IC 106. Of these elements, thecapacitor 101, resistor 102, FET for signal amplification 103 and tuningcircuit 104 constitute an RF amplifier.

[0005] Here, the capacitor 101 is intended to cut a DC component of anAM broadcast signal inputted from an antenna which is not shown and theresistor 102 is intended to give an appropriate bias to the FET forsignal amplification 103. The FET 103 for signal amplification isintended to amplify the inputted AM broadcast signal and is constructedof a junction FET (junction field effect transistor=JFET).

[0006] The tuning circuit 104 is intended to amplify an RF signaloutputted from the FET for signal amplification 103 at a high frequencyand output it to the IC 106 and constructed of a tuning capacitor C1 andtuning coils L1 and L2. One end of this tuning circuit 104 is connectedto a power supply Vcc. On the other hand, the IC 106 is intended toinput the RF amplified signal outputted from the tuning circuit 104 andperform subsequent signal processing necessary for AM broadcastingreception such as mixing and frequency conversion.

[0007] Furthermore, as shown in FIG. 1B, the AM broadcasting receptioncircuit in the non-tuning circuit format is constructed of a capacitor101, a resistor 102, an FET for signal amplification 103, a couplingcapacitor 105, an IC 106 and a coil 107. Of these elements, thecapacitor 101, resistor 102, FET for signal amplification 103, couplingcapacitor 105 and coil 107 constitute an RF amplifier.

[0008] The scale of integration of an RF circuit for a wireless terminalwhich handles high frequency signals of 2.4 GHz band or 5 GHz band,etc., is being increased in recent years and an LSI which incorporatesan RF circuit which has been mounted outside a chip as an individualanalog part so far on a single chip using a CMOS technology is underdevelopment. Likewise, an LSI incorporating an RF circuit using a CMOStechnology for an FM broadcasting receiver using frequency bands of 76Mto 90 MHz is also under development. These RF circuits integrated on asingle chip also include RF amplifiers.

[0009] On the other hand, as shown in FIG. 1, an AM broadcastingreceiver uses a junction (bipolar) type JFET 103 for the RF amplifierand its manufacturing process is different from the CMOS technology, andtherefore the AM broadcasting RF amplifier is still mounted outside thechip of the IC 106 as a separate component. This is because influencesof flicker noise (1/f noise) produced inside a MOS semiconductor aretaken into consideration.

[0010] That is, the noise level of flicker noise is inverselyproportional to frequency, and therefore in the case of a wirelessterminal handling a high frequency signal, almost no flicker noise isproduced even if its RF amplifier is constructed of a CMOS circuit.However, in the case of an AM broadcasting receiver handling lowfrequency signals of medium frequency such as 530 to 1710 KHz or lowfrequency such as 153 to 279 KHz, their frequency bands still belong toareas with large flicker noise components, and therefore it is notdesirable to construct an RF amplifier with a CMOS circuit.

[0011] For this reason, the JFET 103 has been conventionally used forthe RF amplifier. Moreover, an RF amplifier constructed of the JFET 103combined with a bipolar transistor has also been used. However, theseconventional technologies are unable to integrate an RF amplifiertogether with another RF circuit, etc., on a single chip, and as aresult, the conventional technologies have a problem that it is notpossible to reduce the size of an entire circuit as in the case of ahigh frequency wireless terminal.

[0012] The present invention has been implemented to solve theseproblems and it is an object of the present invention to integrate an RFamplifier for AM broadcasting together with other circuits on one chipwhile suppressing the flicker noise to a lowest possible level andthereby realize small size and low noise of the entire circuit.

SUMMARY OF THE INVENTION

[0013] The amplifier circuit for AM broadcasting according to thepresent invention is an amplifier circuit for AM broadcasting foramplifying an inputted AM broadcast signal by FETs and outputting it,characterized in that the FETs are constructed of P-channel MOSFETs.

[0014] In another mode of the present invention, the P-channel MOSFETsare characterized by including a first P-channel MOSFET for amplifyingthe inputted AM broadcast signal and a second P-channel MOSFET forcarrying out AGC control on the signal outputted from the firstP-channel MOSFET.

[0015] A further mode of the present invention is characterized byincluding a first P-channel MOSFET for amplifying the inputted AMbroadcast signal, a second P-channel MOSFET for carrying out AGC controlon the signal outputted from the first P-channel MOSFET and a tuningcircuit for high-frequency amplifying a signal outputted from the secondP-channel MOSFET and outputting it.

[0016] A still further mode of the present invention is characterized byincluding a capacitor for cutting a DC component of an inputted AMbroadcast signal, a first P-channel MOSFET for amplifying the AMbroadcast signal outputted from the capacitor, a resistor for giving anappropriate bias to the first P-channel MOSFET, a second P-channelMOSFET for carrying out AGC control on the signal outputted from thefirst P-channel MOSFET and a tuning circuit for high-frequencyamplifying the signal outputted from the second P-channel MOSFET andoutputting it.

[0017] A still further mode of the present invention is characterized inthat the first P-channel MOSFET and the second P-channel MOSFET arecascode-coupled.

[0018] A still further mode of the present invention is characterized inthat the area of the P-channel MOSFET is larger than a predeterminedvalue.

[0019] According to the present invention configured as shown above, anRF amplifier for AM broadcasting is constructed even in a low frequencyarea using P-channel MOSFETs which involves smaller flicker noise thanN-channel MOSFETs, making it possible to integrate more circuitsincluding the RF amplifier for AM broadcasting on one chip with a MOSstructure while suppressing the flicker noise to a lowest possible leveland thereby realize small size and low noise of the circuits.

[0020] Furthermore, according to another feature of the presentinvention, the channel area of P-channel MOSFETs is increased, whichsuppresses the flicker noise to a further smaller level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIGS. 1A and 1B show a configuration of a conventional AMbroadcasting reception circuit;

[0022]FIG. 2 shows a configuration example of an amplifier circuit forAM broadcasting according to this embodiment; and

[0023]FIG. 3 shows a flicker noise characteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] With reference now to the attached drawings, an embodiment of thepresent invention will be explained below.

[0025]FIG. 2 shows a configuration of an amplifier circuit for AMbroadcasting according to this embodiment. As shown in FIG. 2, theamplifier circuit for AM broadcasting of this embodiment is constructedof a capacitor 1, resistors 2 and 3, a first P-channel MOSFET 4, asecond P-channel MOSFET 5 and a tuning circuit 6.

[0026] Here, the capacitor 1 is intended to cut a DC component of an AMbroadcast signal inputted from an antenna which is not shown. Theresistors 2 and 3 are intended to give an appropriate bias to the firstP-MOSFET 4, connected in series between a power supply Vcc and groundand the capacitor 1 is connected at an intermediate node thereof.

[0027] The first P-MOSFET 4 is intended to amplify the AM broadcastsignal outputted from the capacitor 1. Its gate is connected to thecapacitor 1 at the intermediate node between the resistors 2 and 3, itssource is connected to the power supply Vcc and its drain is connectedto the source of the second P-MOSFET 5.

[0028] The second P-MOSFET 5 is intended to carry out AGC (Auto GainControl) control on the RF signal outputted from the first P-MOSFET 4.Its gate is connected to a power supply for the AGC voltage, its sourceis connected to the drain of the first P-MOSFET 4 and its drain isconnected to the tuning circuit 6. By the way, the power supplyconnected to the gate of the second P-MOSFET 5 need not necessarily bethe power supply for the AGC voltage, but may also be a power supplywith a fixed voltage.

[0029] The tuning circuit 6 is intended to amplify the AGC-controlled RFsignal outputted from the second P-MOSFET 5 at a high frequency andoutput it and is constructed of a tuning capacitor C1 and tuning coilsL1 and L2. One end of this tuning circuit 6 is connected to the drain ofthe second P-MOSFET 5 and the other end is grounded.

[0030] The amplifier circuit for AM broadcasting in the above-describedconfiguration of this embodiment is integrated on one chip together withsubsequent circuits that perform signal processing necessary for AMbroadcasting reception including mixing, frequency conversion, etc., andthe output signal of the tuning circuit 6 is supplied to, for example, amixer stage which is not shown.

[0031] Next, the operation of the amplifier circuit for AM broadcastingconfigured as shown above will be explained. First, a DC component of anAM broadcast signal inputted from an antenna (not shown) is cut by thecapacitor 1 and its output signal is amplified by the first P-MOSFET 4.Then, the RF signal outputted from the first P-MOSFET 4 isAGC-controlled to a certain level by the second P-MOSFET 5 and outputtedto the tuning circuit 6.

[0032] Thus, in this embodiment, the first P-MOSFET 4 for signalamplification and the second P-MOSFET 5 for AGC control arecascode-coupled and the AM broadcast signal is cascode-amplified. Thiscan reduce inter-electrode capacitance and thereby drastically reducefeedback from the output to the input, which provides an excellent highfrequency characteristic. Furthermore, this cascode-coupling is suitablefor AGC control and can increase stability of the circuits.

[0033] Furthermore, the tuning circuit 6 amplifies the RF signal at acertain level outputted from the second P-MOSFET 5 at a high frequencyand outputs it to the next mixer which is not shown. The subsequentsignal processing circuits (not shown) including a mixer and frequencyconversion section carry out remaining processing necessary for AMbroadcasting reception, tune for the input signal, and the output stageperforms amplification and detection, etc., and outputs a speech signal.

[0034]FIG. 3 shows flicker noise characteristics of the P-MOSFET andother MOSFETs used for the amplifier circuit for AM broadcasting of thisembodiment.

[0035] As shown in FIG. 3, the level of the flicker noise, which isinternal noise of the MOS semiconductor, increases in inverse proportionto the frequency. Therefore, when the signal handled is a low frequencysignal such as AM broadcast signal, constructing the RF amplifier with aMOS circuit increases the noise level compared to when JFET is used.

[0036] However, when an N-MOSFET is compared with a P-MOSFET, the noiselevel of the P-MOSFET is lower than that of the N-MOSFET even in a lowfrequency area. This embodiment constructs the FETs 4 and 5 for signalamplification and AGC control with only P-MOSFETs which are neitherN-MOS nor CMOS, and can thereby suppress the flicker noise to arelatively low level.

[0037] On top of it, P-MOS technology and CMOS technology have the samemanufacturing process, and can thereby integrate the entire RF circuitfor AM broadcasting including the amplification circuit of thisembodiment on one chip and reduce the size of the entire circuit.Furthermore, the ability to manufacture the entire RF circuit throughthe same MOS process can simplify the manufacturing process and reducemanufacturing costs. Of course, not only the RF circuit but also thesubsequent baseband circuit, etc., can be integrated on one chip.

[0038] Next, the idea for further reducing flicker noise will beexplained below. With the first and second P-MOSFETs 4 and 5, increasingthe area of the channel through which a current (or carrier) flows canfurther suppress flicker noise.

[0039] In this case, it is possible to increase either one of thechannel width or channel length of the FET, but it is preferable toincrease both. When an RF amplifier of a wireless terminal which handleshigh frequency signals is constructed with a CMOS circuit, MOSFETs witha channel width and channel length of approximately 0.7 μm×1.5 μm, 0.6μm×1.4 μm or 0.2 μm×1.0 μm are currently used. As the RF amplifier forAM broadcasting of this embodiment, it is preferable to use P-MOSFETs 4and 5 having a larger channel area than this. For example, the channelwidth can be set to 1000 μm and the channel length can be set to 2 μm.

[0040] By the way, the above-described embodiment is not more than aspecific example in implementing the present invention and this shouldnot be interpreted as restricting the technological scope of the presentinvention. That is, the invention may be embodied in other specificforms without departing from the spirit or essential characteristicthereof. For example, the present invention is applicable not only to atuning circuit format but also to a non-tuning circuit format.

INDUSTRIAL APPLICABILITY

[0041] The present invention is useful to realize small size and lownoise of the entire circuit by integrating an RF amplifier for AMbroadcasting together with other circuits on one chip while suppressingflicker noise to a lowest possible level.

What is claimed is:
 1. 2.
 3. 4.
 5. An amplifier circuit for AMbroadcasting for amplifying an inputted AM broadcast signal by FETs andoutputting it, characterized in that said FETs include a first P-channelMOSFET for amplifying said inputted AM broadcast signal and a secondP-channel MOSFET for AGC-controlling the signal outputted from saidfirst P-channel MOSFET; wherein said first P-channel MOSFET and saidsecond P-channel MOSFET are cascode-coupled.
 6. An amplifier circuit forAM broadcasting, characterized by comprising: a first P-channel MOSFETfor amplifying an inputted AM broadcast signal; a second P-channelMOSFET for AGC-controlling the signal outputted from said firstP-channel MOSFET; and a tuning circuit for high-frequency amplifying thesignal outputted from said second P-channel MOSFET and outputting it;wherein said first P-channel MOSFET and said second P-channel MOSFET arecascode-coupled.
 7. An amplifier circuit for AM broadcasting,characterized by comprising: a capacitor for cutting a DC component ofan inputted AM broadcast signal; a first P-channel MOSFET for amplifyingthe AM broadcast signal outputted from said capacitor; a resistor forgiving said first P-channel MOSFET an appropriate bias; a secondP-channel MOSFET for AGC-controlling the signal outputted from saidfirst P-channel MOSFET; and a tuning circuit for high-frequencyamplifying the signal outputted from said second P-channel MOSFET andoutputting it; wherein said first P-channel MOSFET and said secondP-channel MOSFET are cascode-coupled.
 8. The amplifier circuit for AMbroadcasting according to claim 5, characterized in that the channelarea of said P-channel MOSFET is greater than a predetermined value. 9.The amplifier circuit for AM broadcasting according to claim 6,characterized in that the channel area of said P-channel MOSFET isgreater than a predetermined value.
 10. The amplifier circuit for AMbroadcasting according to claim 7, characterized in that the channelarea of said P-channel MOSFET is greater than a predetermined value.